1. Field of the Invention
The present invention generally relates to the field of switching mode power converters. More particularly, the present invention relates to PFC-PWM controllers.
2. Description of the Prior Art
The purpose of Power Factor Correction (PFC) is to correct a line current of a power supply. Power Factor Correction produces a sinusoidal input current waveform that is in phase with a line input voltage. With a PFC circuitry in a front-end of the power supply, a DC-to-DC converter can significantly reduce power loss and heat dissipation in power contribution systems.
Recently enacted environmental regulations in the U.S. and in Europe not only require most consumer products to have built-in PFC function, but also strictly limit overall power consumption. Specifically, the amount of power consumption permitted for supervising and remote control purposes has been significantly reduced. Therefore, reducing power consumption under standby mode becomes a major concern among electronics manufacturers.
Traditional DC-to-DC converters with PFC function still have high power consumption under light-load and zero-load conditions. Because of this, many present-day electronic product designs are not compliant with power conservation requirements. Furthermore, when the PFC circuitry is cascaded with PWM (pulse width modulation) circuitry, significant switching interference and EMI (electrical-magnetic interference) could occur. To alleviate these problems, most DC-to-DC converters incorporate a PWM circuitry having some form of synchronous switching.
One method of synchronizing PFC and PWM signals is described in U.S. Pat. No. 5,565,761 (Hwang). Hwang discloses a leading-edge and trailing-edge modulation technique, in which the PFC boost converter switches (the first stage) and the DC-to-DC power converter switches (the second stage) are turned on and off at the same time. This minimizes the duration of the temporary zero-load period and reduces the magnitude of the ripple voltage delivered to the load.
However, one drawback of Hwang's invention is that power consumption is not reduced under light-load and zero-load conditions. Another drawback of Hwang's invention is poor output response to dynamic loads because of the duty cycle of the second stage being not directly controlled by the output voltage.
Furthermore, Hwang's invention teaches a DC-to-DC power converter having a dc ok comparator coupled to the first stage. The dc ok comparator prevents the second stage from turning on if the output voltage of the first stage is below a threshold value. However, the dc ok comparator is sensitive to noise interference. Spike and overshoot signals can incorrectly turn on the second stage.
Another drawback of Hwang's invention is that it generates significant noise and EMI during leading edge and trailing edge switching. To minimize ripple voltage, the PFC boost converter switches and the DC-to-DC converter switches are turned on and off at the same time. However, this technique mutually modulates the switching noise, and doubles its magnitude. Furthermore, the PFC-PWM controller according to Hwang simultaneously conducts the parasitic devices of the PFC and PWM stages. This can result in the creation of a multi-resonant tank that generates substantial high frequency noise.
The objective of the present invention is to provide a PFC-PWM controller that overcomes the drawbacks of the prior art. The present invention also includes a means for reducing power consumption while the power converter is operating in standby mode.